Heat pumps are used in a variety of settings, for example, in HVAC systems that provide a desired air temperature in a facility. Such heat pumps commonly include a compressor, evaporator, expansion valve, and condenser. The heat pumps input work to the refrigerant, e.g., by driving the compressor, thereby enabling the refrigerant to move heat from a colder heat reservoir to a warmer heat sink.
Some heat pumps are provided as “split” systems, in which the condenser (in heating applications) is disposed inside of the facility, while compressor, evaporator, and expansion valve are disposed outside the facility. This allows for efficient moving of heat from the outside (reservoir) to the inside (sink).
“Frosting” of the outside unit is a common problem seen in such heat pump split systems when implemented in colder climates. Frosting is caused by moisture accumulation on the evaporator, typically in temperatures just above freezing, for example, between 0° C. and 5° C. The accumulated moisture is then frozen by the cold refrigerant coursing through the evaporator and obstructs the flow of air past the evaporator, which reduces operating efficiency. Frosting can also be seen in warmer, humid climates, where the heat pump is configured to cool the facility and the evaporator is disposed inside the facility, while the condenser is outside.
One way in which frosting is avoided is by providing periodic defrost cycles in the heat pump. The defrost cycle typically proceeds by reversing the flow of the refrigerant in the heat pump, such that the condenser and evaporator conceptually switch places. The result is that the refrigerant warms the evaporator, thereby avoiding such frost accumulation.
However, initiating and terminating defrost cycles by reversing the refrigerant flow presents challenges. Efficient operation of the heat pump relies on a relatively high compression ratio between the high-pressure side (downstream of the compressor and upstream of the expansion valve) and the low-pressure side (upstream of the compressor and downstream of the expansion valve). With such a high pressure differential, reversing the refrigerant flow to initiate or terminate a defrost cycle often results in a rush of reversing refrigerant flowing from the high-pressure side to the low-pressure side. This can result in valve screech, “groaning,” “swooshing,” and a variety of other noises that give users the impression that high quality, working parts are either broken or poorly made.
What is needed is a method for reducing perceived defrost noises in a heat pump.